The main objective of the proposed research is to determine the manner and the extent to which higher-level units, such as words and familiar visual scenes, facilitate encoding and stimulus identification. In one task, the subject will monitor a very rapid sequence of words or nonwords (scrambled collections of letters) for a target letter; in another case, the subject will decide whether a familiar (upright) or unfamiliar (upside-down) test picture had previously been shown. A second objective is to study several comparison procedures currently being used to measure the effect of familiarity on visual information processing. The intent is to separate out nonperceptual factors. Tests will be made to determine whether processing at the pre-comparison stimulus encoding stage is better revealed when the stimuli are more symmetrical, more discriminable, and more highly overlearned.